Maximizing a window overlaps gkrellm and xfce4-panel

Hello.

I use gkrellm as a system status monitor; when I used sawfish, it was configured so that when maximizing windows gkrellm wouldn't disappear under them, by reserving a blank column on the right side of the desktop, of about 48 pixels, when windows didn't go when maximized.

However I can't replicate this behavior with xfwm4: maximized windows take up all the screen, even the space where xfce4-panel is.

I tried creating rules with devilspie2 to declare gkrellm as a dock, so that it would be avoided when maximizing, but it doesn't work.